Osteoporosis, while recognized as a major disorder of postmenopausal women, can also be seen in middle-aged men. While some of the men who present with evidence of skeletal thinning have disorders or behaviors that can explain their osteoporois, others have no apparent cause. The appropriate treatment of this small but important group of patients is unknown. One agent that potentially could be useful in these patients is biologically active parathyroid hormone (hPTH[1-34]), an agent known to stimulate bone formation. This drug has shown promise in postmenopausal women with osteoporosis, and also in a small, uncontrolled study of men with idiopathic osteoporosis. It is the aim of this study to investigate the efficacy of hPTH[1-34] in men with idiopathic osteoporosis. This is a double-blind, placebo-controlled, randomized protocol to investigate the efficacy and safety of hPTH[1-34], as a therapy for idiopathic osteoporosis in middle-aged men. After a six-month pre-treatment period, subjects will self-administer by subcutaneous injection drug or placebo for a period of 1.5 years. The study evaluates bone mass as the primary outcome variable over a period of eighteen months.